Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of a catalyst. This gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of polymer that cannot be removed as product. Further, such chunks or sheets may fall onto the distributor plate causing impaired fluidization, and in many cases forcing a reactor shutdown. The prevention of such stickiness has been accomplished through controlling the temperature of the gaseous stream in the reaction bed to a temperature below the fusion or sintering temperature of the polymer particles produced during the polymerization reaction. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to agglomeration or stickiness, which in turn can, if left unchecked, may lead to the above conditions.
It is understood that the amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from a reaction zone in a fluidized bed within the reactor. Conventionally, heat has been removed from the gaseous recycle stream by cooling the recycle stream outside the reactor. A requirement of a fluidized bed process is that the velocity of the gaseous recycle stream be sufficient to maintain the reaction zone in a fluidized state. In a conventional fluidized bed polymerization process, the amount of fluid circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate mixing of the solids in the fluidized bed. The excess velocity provides additional gas flow to (and through) the fluid bed for additional cooling capacity and more intensive mixing of the reactor bed. However, to prevent excessive entrainment of solids in a gaseous stream withdrawn from the fluidized bed, the velocity of the gaseous stream must be regulated. Also, in a steady state fluidized bed polymerization process wherein the heat generated by the polymerization reaction is proportional to the rate of polymer production, the heat generated is equal to the heat absorbed by the gaseous stream and lost by other means, such that the bed temperature remains constant.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point can be calculated knowing the gas composition and is thermodynamically defined using an equation of state. It was believed that condensed liquid in the recycle stream would inevitably result in plugging of the recycle stream lines, the heat exchanger, the area below the fluidized bed and especially the gas distributor plate. As a consequence of operating at a temperature above the dew point of the recycle stream to avoid the expected problems associated with liquid being in the gaseous recycle stream, production rates in commercial reactors could not be significantly increased without enlarging reactor diameters for increased heat removal capability. There was also concern that excessive amounts of liquid in the recycle stream would disrupt the fluidization process to the extent that the fluidized bed would collapse resulting in the sintering of solid polymer particles into a solid mass causing the reactor to shut down.
Contrary to this belief, as suggested by Jenkins, et al. in U.S. Pat. No. 4,543,399 and related U.S. Pat. No. 4,588,790 a recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream. The disclosures of these two Jenkins patents are incorporated herein by reference. The resulting stream containing entrained liquid is then returned to the reactor without causing the aforementioned agglomeration and/or plugging phenomena (which had been expected prior to Jenkins). This process of purposefully condensing a portion of the recycle stream is known in the industry as a “condensed mode” operation in a gas phase polymerization process.
The above-mentioned U.S. patents to Jenkins et al. suggest that when a recycle stream temperature is lowered to a point below its dew point in “condensed mode” operation, an increase in polymer production is possible, as compared to production in a non-condensing mode because of increased cooling capacity. Also, Jenkins, et al. found that a substantial increase in space time yield, the amount of polymer production in a given reactor volume, can be achieved by operating in “condensed mode” with little or no change in product properties.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture with solids contained in both of these phases. The liquid phase of this two-phase gas/liquid mixture in “condensed mode” operation remains entrained or suspended in the gas phase of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. In the process described by Jenkins, et al., vaporization occurs when the two-phase mixture enters the reactor. The increase in space time yields achieved by Jenkins, et al. are the result of this vaporization, and also the increased temperature differential between the entering recycle stream and the fluidized bed temperature. Both of these factors increase the heat removal capability of the system and thereby enable higher space time yields (higher reactor production rates).
Jenkins, et al. illustrate the difficulty and complexity of such reactor control in general, and of trying to extend the stable operating zone to optimize the space time yield in a gas phase reactor, especially when operating in condensed mode.
The cooling capacity of the recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. One option described is to add non-polymerizing, non-reactive materials to the reactor, which are condensable at the temperatures encountered in the process heat exchanger. Such non-reactive, condensable materials are collectively known as induced condensing agents (ICAs). Increasing concentrations of ICA in the reactor causes corresponding increases in the dew point temperature of the reactor gas, which promotes higher levels of condensing for higher (heat transfer limited) production rates from the reactor. Suitable ICA materials are selected based on their specific heat and boiling point properties. In particular, the ICA compound is selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are typically 20-40° C. Such ICA materials include hexane, isohexane, pentane, isopentane, butane and isobutane.
Recognition that high concentrations of condensable gases, either ICA, comonomers or combinations thereof, can cause the polymer in the fluid bed to become sticky can be found in U.S. Pat. No. 5,352,749 (DeChellis). When either or both of these condensable materials are present, the reactor contents, specifically the polymer contents, can become sticky because these condensable materials are highly soluble in the polymer particles. When the condensable concentrations are higher than acceptable levels they effectively lower the polymer melting point, and stickiness can result. The same effect can be the result of increasing reactor temperature in the absence of excess concentrations of condensable components, or the result of combinations of increased reactor temperature and high concentrations of condensable components (either or both of ICA and/or comonomer).
In the '749 document, and in U.S. Pat. Nos. 5,405,922 and 5,436,304, the ICA is at relatively high concentrations to thereby increase heat removal capacity of the system based on the latent heat of vaporization associated with the ICA or liquid monomer. Upper limits of ICA in the reactor are discussed, depending on the type of polymer being produced. Attempts to go beyond such levels caused the fluid bed or, more specifically, the polymer particles suspended in the fluid bed, to become cohesive or “sticky”, and in some cases caused the fluid bed to solidify in the form of a large chunk. The stickiness problem is characterized by undesirable changes in fluidization and mixing in the fluid bed, which if left unchecked, may develop into a reactor discontinuity event, such as sheeting in the straight sided reaction section, sheeting in the dome of such a reactor or chunking, any of which can lead to a reactor shut-down, which in large scale reactors are expensive. These solid masses of polymer (the sheets or chunks) eventually become dislodged from the walls and fall into the reaction section and settle on the distributor plate, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning, any one of which can be termed a “discontinuity event”, which in general is a disruption in the continuous operation of a polymerization reactor. The terms “sheeting and/or chunking” while used synonymously herein, may describe different manifestations of similar problems, in each case they can lead to a reactor discontinuity event.
In U.S. Pat. No. 5,352,749, the authors determined that a limiting concentration of ICA (isopentane) existed, beyond which the reactor contents would suddenly loose fluidization. The authors characterized this limit by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane was increased, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They therefore determined that this ratio (0.59) was a point of no return, below which the reactor will cease functioning due to loss of fluidization. Although this was not appreciated by the authors of U.S. Pat. No. 5,352,749, the sudden loss in fluidization at relatively high ICA concentrations was due to the formation of sticky polymer.
Two articles by Process Analysis & Automation Limited (PAA) (“Agglomeration Detection by Acoustic Emission” PAA Application note: 2002/111 © 2000; and “Acoustic Emission Technology—a New Sensing Technique for Optimising Polyolefin Production” © 2000), suggest process control in fluidized bed production of polyolefins utilizing acoustic emission sensors located at various positions on the reactor and recycle piping. These publications purport to solve the problem of detection of large polymer agglomerates in the reactor, such as chunks or sheets, rather than the detection of stickiness of the resin particles, as provided by embodiments of the present invention. The PAA documents provide only one specific example, showing the detection of a chunk of approximately 1.5 meters in diameter within a commercial fluid bed reactor. There is no mention of the detection of polymer stickiness or cohesiveness. In effect, the PAA documents describe the detection of agglomerates after they have been formed in the reactor, rather than the detection of resin stickiness that, if left unchecked, could lead to the formation of the agglomerates. Additionally, the PAA documents suggest that the presence of agglomerates is indicated by an increase in acoustic emission signal intensity (presumably by recording the noise generated when the relatively large chunks of polymer strike the walls of the reactor). In contrast, embodiments of the present invention teach that increasing levels of resin stickiness are indicated as decreases in acoustic emission signal intensity.
Chinese application 200310113358.7 purports to solve the problem of determining particle size distribution through (acoustic) signal decomposition. Detection of large agglomerates is discussed, of 22 mm particle size. No specific corrective action is suggested to address the problem (or problems) that create the polymer agglomerates.
WO 03/051929 (incorporated herein by reference), describes the use of mathematical chaos theory to detect the onset and presence of sheeting in a fluid bed reactor. Signals from a range of instruments, including acoustic emission sensors, differential pressure sensors, static sensors, and wall temperature sensors are filtered by certain specified methods to construct a “time-series” of data, which is then processed by methods of non-linear dynamics herein referred to as chaos theory and compared to data from a control reactor running without sheeting. The onset of sheeting is indicated by an increase in mean “cycle time” (relative to the control reactor), usually with a concurrent decrease in the “mean deviation” of the time-series. Alternatively, the onset of sheeting is indicated by a decrease in the mathematical “entropy” of the time-series data, as compared to a similar reactor running without sheeting. (The terms “time-series”, “cycle time”, “mean deviation”, and “entropy” refer to calculated parameters defined by chaos theory.) There is no disclosure of the use of acoustic emission sensors to determine conditions of impending stickiness in the reactor, nor is there a disclosure of the use of these sensors to provide information relevant to distributor plate fouling. There is no disclosure to the use of simple averages and standard deviations of the acoustic emission readings (without recourse to the complexities involved with chaos theory), and there is no disclosure of the use of these sensors (or any other methods) to allow safe operation of a reactor near its limit of ultimate cooling capacity for maximum production rates.
Adding to the complexity of control of stickiness while using ICAs, different polymer products vary widely in their ability to tolerate such ICA materials, some having a relatively high tolerance (expressed in partial pressure of the ICA in the reactor), e.g. 50 psia, while other polymers may tolerate as little as 5 psia, and in these latter polymers, the heat transfer limited production rates, under similar conditions, are substantially lower. Polymers which possess a more uniform comonomer composition distribution are known to have a higher tolerance to the partial pressure of the ICA in the reactor. Metallocene catalyst produced polymers are a good example of polymers with such a more uniform comonomer composition. However, at some point even these metallocene produced polymers reach a limiting ICA concentration that induces stickiness. The limiting ICA concentration depends on several factors in addition to the polymer type, and include reactor temperature and comonomer type and concentration. Further, with the effect of temperature, ICA level and comonomer levels all affecting on the onset of stickiness, determining the point at which sticking begins to occur has been heretofore been difficult.
Even within the constraints of conventional, safe operation, control of such reactors is complex adding further to the difficulty and uncertainty of experimentation if one wishes to find new and improved operating conditions that might result in higher production rates.
Large scale gas phase plants are expensive and highly productive. Risks associated with experimentation in such plants are high because downtime is costly. Therefore it is difficult to explore design and operating boundaries experimentally in view of the costs and risks.
It would be desirable to provide a method of determining a stable operating condition for gas fluidized bed polymerization, especially if operating in condensed mode, to facilitate optimum design of the plant and the determination of desirable process conditions for optimum or maximum production rates in a given plant design.
It would also be desirable to have a mechanism in commercial gas-phase reactors to detect the onset of stickiness that is a better indicator, or an earlier indicator of the onset of stickiness than the conventional technique of monitoring the fluidized bulk density (as described in U.S. Pat. No. 5,352,749) or other methods, allowing operators to determine when excessive sticking is occurring and enabling those operators to take corrective action, while keeping the reactors at or near this optimum production rate point, permitting higher production rates with substantially less risk.